This invention relates generally to a method for making high density interconnect-compatible magnetic circulators.
Microwave components typically include active and passive devices which are formed to provide various types of microwave circuits. One such particular application is in a transmit/receive module (transceiver module) for use in phased array antennas. In these transceiver modules, active devices such as field effect transistors are combined with passive devices such as capacitors, resistors, inductive elements, and the like to form various microwave functions such as amplifiers and switches. In certain applications, it would be desirable to provide one or more circulators to steer electromagnetic energy through the transceiver modules.
Although non-reciprocal microwave components such as circulators have been known for many years, no convenient approach is available for combining the circulator function into a multi-chip module incorporating a high density interconnect (HDI) structure.
The interconnect structure used in the fabrication of high density interconnect (HDI) circuits has many advantages in the compact assembly of MCMs. For example, a multi-chip electronic system (such as a microcomputer incorporating 30-50 chips) can be fully assembled and interconnected by a suitable HDI structure on a single substrate, to form a unitary package which is 2 inches long by 2 inches wide by 0.050 inches thick. Even more important, the interconnect structure can be disassembled from the substrate for repair or replacement of a faulty component and then reassembled without significant risk to the good components incorporated within the system. This is particularly important where many (e.g., 50) chips, each being very costly, may be incorporated in a single system on one substrate. This repairability feature is a substantial advance over prior connection systems in which reworking the system to replace damaged components was either impossible or involved substantial risk to the good components.
Briefly, in this high density interconnect structure, a ceramic substrate such as alumina which may be 50-100 mils thick and of appropriate size and strength for the overall system, is provided. This size is typically less than 2 inches square, but may be made larger or smaller. Once the position of the various chips has been specified, individual cavities or one large cavity having appropriate depth at the intended locations of differing chips, is prepared. This may be done by starting with a bare substrate having a uniform thickness and the desired size. Conventional, ultrasonic or laser milling may be used to form the cavities in which the various chips and other components will be positioned. For many systems where it is desired to place chips nearly edge-to-edge, a single large cavity is satisfactory. That large cavity may typically have a uniform depth where the semiconductor chips have a substantially uniform thickness. The cavity bottom may be made respectively deeper or shallower at a location where a particularly thick or thin component will be placed, so that the upper surface of the corresponding component is in substantially the same plane as the upper surface of the rest of the components and the portion of the substrate which surrounds the cavity. The bottom of the cavity is then provided with a thermoplastic adhesive layer, which may preferably be a polyetherimide resin (such as ULTEM.RTM. 6000 resin, available from the General Electric Company, Fairfield, Conn.), or an adhesive composition such as is described in U.S. Pat. No. 5,270,371, herein incorporated in its entirety by reference. The various components are then placed in their desired locations within the cavity and the entire structure is heated to remove solvent and thermoplastically bond the individual components to the substrate.
Thereafter, a film, which may be "KAPTON.RTM." polyimide, (available from E. I. du Pont de Nemours Company, Wilmington, Del.), of a thickness of approximately 0.0005-0.003 inches (approx. 12.5-75 microns), is pre-treated by reactive ion etching (RIE) to promote adhesion. The substrate and chips must then be coated with ULTEM.RTM. 1000 polyetherimide resin or another thermoplastic adhesive to adhere the KAPTON.RTM. resin film when it is laminated across the tops of the chips, any other components and the substrate. Thereafter, via holes are provided (preferably by laser drilling) through the KAPTON.RTM. resin film, and ULTEM.RTM. resin layers, at locations in alignment with the contact pads on the electronic components to which it is desired to make contact. A multi-sublayer metallization layer, with a first sublayer comprising titanium (approximately 1000 .ANG.) and a second layer comprising copper (approximately 2000 .ANG.), is sputter deposited over the KAPTON.RTM. resin layer and extends into the via holes to make electrical contact to the contact pads disposed thereunder. The sputtered copper provides a seed layer for copper electroplating (3 to 4 microns thick). A final layer of titanium (1000 .ANG.) is sputter deposited to complete the Ti/Cu/Ti multilayer metallization. This metallization layer is patterned to form individual conductors using photoresist and etching. The photoresist is preferably exposed using a laser to provide an accurately aligned conductor pattern at the end of the process. Alternatively, exposure through a mask may be used.
Additional dielectric and metallization layers are provided as required in order to provide all of the desired electrical connections among the chips. Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system which is the subject of some of the patents and applications listed hereinafter.
This high density interconnect structure provides many advantages. Included among these are the lightest weight and smallest volume packaging of such an electronic system presently available. A further, and possibly more significant, advantage of this high density interconnect structure, is the short time required to design and fabricate a system using this high density interconnect structure. Prior art processes require the prepackaging of each semiconductor chip, the design of a multilayer circuit board to interconnect the various packaged chips, and so forth. Multilayer circuit boards are expensive and require substantial lead time for their fabrication. In contrast, the only thing which must be specially pre-fabricated for the HDI system is the substrate on which the individual semiconductor chips will be mounted. This substrate is a standard stock item, other than the requirement that the substrate have appropriate cavities therein for the placement of the semiconductor chips so that the interconnect surface of the various chips and the substrate will be in a single plane. In the HDI process, the required cavities may be formed in an already fired ceramic substrate by conventional or laser milling; this process is straight-forward and fairly rapid.
The high density interconnect structure, methods of fabricating it and tools for fabricating it are disclosed in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 5,127,998, entitled "Area-Selective Metallization Process" by H. S. Cole et al.; U.S. Pat. No. 5,127,998, entitled "Area-Selective Metallization Process" by H. S. Cole, et al.; and U.S. Pat. No. 5,108,825, entitled "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" by Wojnarowski, et al; U.S. Pat. No. 5,300,812, entitled "Plasticized Polyetherimide Adhesive Composition and Usage" by Lupinski et al; and U.S. Pat. No. 5,206,712, entitled "Building Block Approach to Microwave Modules" by Kornrumpf et al. Each of these Patents and Patent Applications, including the references contained therein, is hereby incorporated in its entirety by reference.
Commercially available discrete magnetic circulators, packaged in a metal can, are readily available and are used in microwave circuits. However, trying to incorporate one of these circulators into a microwave HDI structure is problematic. For example, these commercially available circulators have a profile (height) which is not compatible with a HDI multi-chip module. This high profile presents several difficulties when trying to incorporate the presently available circulators into a high density interconnected multi-chip module. The commercially available circulators must be attached on a substrate using a conductive polymer adhesive. This causes signal loss due to transmission line discontinuity between the ground plane and voids in the conductive polymer adhesive. In addition, the non-hermiticity of the circulator component also causes a final hermetic assembly problem of the circulator/HDI module combination. Testability is another problem to be solved because the final microwave module cannot be tested unless the circulator component is attached to the module.
Consequently, an improved method for implementing a circulator into a high density interconnect multi-chip module, is desirable.